In diesel engines, to reduce combustion noise and clean up exhaust property, a pilot injection, in which a small amount of fuel is injected prior to a main injection, and an after injection, in which a small amount of fuel is injected again after the main injection, are performed. In such pilot injection and after injection, minute fuel injection amounts need to be precisely adjusted. However, since the fuel injection property of injectors varies due to the initial individual difference, changes over time, or the state of injection holes that varies moment to moment, slight divergence is inevitable between the required fuel injection amount set in accordance with the engine operating state and the amount of fuel actually injected from the injectors. In particular, when controlling the minute fuel injection amount such as the pilot injection and the after injection, although such divergence between the required fuel injection amount and the actual fuel injection amount is small, its influence cannot be ignored.
In the conventional art, for example, a fuel injection control device as disclosed in Patent Document 1 has been proposed. The device executes multiple injection in which fuel is equally divided and injected so as to correct the fuel injection property of injectors, in particular, the fuel injection property when a minute amount of fuel is injected.
In this device, when a predetermined learning condition is satisfied during idle operation of the engine, first, the fuel injection mode is shifted to the multiple injection. The fuel injection amount is then adjusted to eliminate the fluctuation of the engine rotational speed and the fluctuation among cylinders so that the engine operating state is stabilized. Since the total minimum fuel injection amount required to execute stable engine idle operation is almost uniquely determined in accordance with, for example, the specification of the engine, when the total minimum fuel injection amount is represented by “Q”, for example, the fuel injection amount in the multiple injection is expressed by “Q/n (n: number of divisions)”. When the injection command signal of an injector in a state where the engine operating state is stabilized is “T”, that is, when the time during which the injector is supplied with power in each injection of the multiple injection is “T”, the time during which the injector is supplied with power appropriate for injecting fuel the amount of which is “Q/n” is “T”. Thus, the time T is learned as the injection command signal corresponding to the fuel injection amount Q/n. Since such learning is carried out, even if the fuel injection property of the injector deviates from the standard fuel injection property due to the initial individual difference and changes over time, the injection command signal of the injector is corrected in accordance with the actual fuel injection property.
However, the following is concerned in the conventional learning process associated with the minute fuel injection amount.
In this learning process, the fuel injection amount when the engine idle operating state is stable needs to be constant regardless of the fuel injection mode.
That is, the fuel injection amount required to obtain stable engine idle operating state needs to be basically the same in both the case where fuel is injected all together, and the case where fuel is equally divided and injected multiple times.
However, in practice, the fuel injection amount required to obtain stable engine idle operating state varies depending on the fuel injection mode between the case where batch injection is executed and the case where multiple injection is executed. This is because of various factors such as that the fuel injection amount of each phase fluctuates by the fuel pressure pulsation caused by executing the multiple injection, or that the generation pattern of the engine output is inevitably changed due to difference in the flame propagation state in the engine combustion chambers.
Therefore, in the conventional fuel injection control device, although the fuel injection amount is corrected in accordance with the actual fuel injection property of the injectors, there is a limitation to the correction accuracy. In view of correcting the fuel injection amount with sufficiently high reliability also in the minute amount fuel injection such as the pilot injection and the after injection, there is room for improvement.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-27995